Polyimides are used in electronic components and electrical machinery components because of their high heat resistance and excellent electrical insulating properties. Transparent polyimides having an aliphatic structure are used also in liquid crystal display elements. However, the introduction of the aliphatic structure to impart transparency to a polyimide can lower the heat resistance and the mechanical characteristics of the polyimide. Thus, a polyimide having high transparency, high heat resistance, and excellent mechanical characteristics has been synthesized by introducing a specific alicyclic structure (see PTL 1). A polyimide having high transparency, high heat resistance, and excellent mechanical characteristics has been synthesized by using a substantially planar diamine, such as trans-1,4-cyclohexanediamine (see PTL 2). However, use of trans-1,4-cyclohexanediamine made polymerization difficult because of the formation of a salt during polymerization. Thus, the formation of a salt must be reduced, for example, by silylation of the diamine.
It is also known that a polyimide produced using pyromellitic acid and 4,4′-methylenebis(aminocyclohexane) has high transparency, high heat resistance, and excellent mechanical characteristics (see PTL 3). However, the polyimide produced by this method has low solubility. Thus, a film of the polyimide must be manufactured by heat treatment of a film of a precursor, such as polyamic acid, at high temperature. This causes problems, such as thermal damage to a substrate and degradation of transparency because of the yellow coloration of the polyimide. Thus, there is a demand for a polyimide that is easy to synthesize, has high transparency and heat resistance, and can be processed without causing thermal damage to neighboring members.
In an antireflective structure having a periodic fine structure having a pitch less than or equal to a wavelength in a visible light region, it is known that the formation of a periodic fine structure having an appropriate pitch and height results in high antireflection performance in a wide wavelength range. A known method for forming a periodic fine structure includes the application of a film in which fine particles having a size less than or equal to the wavelength are dispersed. In particular, it is known that a textured structure formed of aluminum oxide boehmite grown on a glass substrate has a high antireflection effect. This textured structure formed of boehmite is produced by steam treatment or hot-water immersion treatment of an aluminum oxide film, for example, formed by a liquid phase method (a sol-gel method) (see NPL 1). However, exposure to water vapor or hot water can cause damage to the glass substrate.
It is known that polyimides can be transparent, have a variable refractive index, and protect a glass substrate from damage caused by water or water vapor (see PTL 4). However, it is difficult to produce a polyimide that is easy to synthesize and has high transparency and heat resistance. In order to manufacture a low-reflectance optical member, there is a demand for an optical thin film that has small variations in thickness and optical properties.
A porous film that contains fine particles deposited on the surface layer as an antireflection coating and a metal oxide or halogenated metal layer formed by a method of growing boehmite on a substrate are convenient and have high productivity and excellent optical performance. On the other hand, the porous film and the metal oxide or halogenated metal layer have low density and many voids. Thus, water from the outside can easily reach the substrate, often causing erosion of the substrate or the elution of substrate components, such as alkali ions. Thus, there is a demand for a thin-film material that can be applied between a porous film or a boehmite film and a substrate to improve antireflection performance and reduce damage to the substrate. Furthermore, there is a demand for a high-performance antireflection-coated optical member without cracking or film irregularities caused by a variation in film thickness or optical properties resulting from the effects of heat or water.